1. Field of the Invention
This invention relates to field effect transistors and more particularly to a metal semiconductor field effect transistor.
2. Description of the Prior Art
In space and airborne electronics, such as a phased-array radar, an overriding requirement is for high power rf or microwave efficiency. Not only can a significant reduction in weight be achieved through reduced input power and heat dissipation requirements, but improved transistor reliability is obtained through efficient, lower temperature operation.
The rf or microwave power and efficiency delivered by a gallium arsenide metal semiconductor field effect transistor (MESFET) may be conveniently predicted from the DC characteristics of the device. If I.sub.f is the maximum channel current in the field effect transistor, V.sub.K is the saturation or knee voltage, and V.sub.SD.sup.L is the sustainable source-to-drain voltage at or near pinch-off, then the available output power in watts is given by equation (1). ##EQU1## In equation (1), I.sub.m is the leakage current or unmodulated source-to-drain current. The maximum Class A drain efficiency in percent of the field effect transistor is given by equation (2). ##EQU2## For the efficiency at maximum power, V.sub.SD in equation (2) would equal V.sub.SD.sup.L of equation (1). Inspection of equations (1) and (2) shows that a reduction or elimination of I.sub.m and V.sub.K will improve efficiency to a maximum of 50%, and an increase in V.sub.SD.sup.L without an increase in leakage current I.sub.m will enhance the output power according to equation (1) in direct proportion to V.sub.SD.sup.L.
Present technology for gallium arsenide MESFET fabrication provides field effect transistors with DC characteristics typified by substantial source-drain leakage currents and a soft premature breakdown such as discussed in a publication by DiLorenzo, J. V. and Wisseman, W. R., "GaAs Power MESFETs Design Fabrication and Performance," (1979), IEEE Trans. M.T.T. 27, No. 5, p. 367.
The souce-drain leakage currents and soft premature breakdown are accompanied by saturated output power with increased drain voltage and depressed DC-to-rf conversion efficiency as discussed in the publications by Wemple, S. H., Niehaus, W. C., Schlosser, W. O., and Cox, H. M., "Performance of GaAs Power MESFETs," (1978), Electron Lett., Vol. 14, p. 175 and Macksey, H. M., Adams, R. L., McQuiddy, D. N., Jr., Shaw, D. W., and Wisseman, W. R., "Dependence on GaAS MESFET Microwave Performance on Device and Material Parameters," (1977), IEEE Trans. Electron Devices, Vol. ED24, p. 113.
Premature source-drain breakdown has been an important topic in the literature in recent years. Explanations offered for the breakdown mechanism have been numerous. In a publication by Wemple, S. H., Niehaus, W. C., Cox, H. M., DiLorenzo, J. V., and Schlosser, W. O., entitled "Control of Gate to Drain Avalanche in GaAs MESFETs," (1980), and found in IEEE Trans. Electron Devices, No. 6, p. 103, gate to drain avalanche was discussed.
In a publication By Tiwari, W., Eastman, L. F., and Rathburn, L., entitled "Physical and Material Limitations on Burnout Voltage of GaAS Power MESFETs," found in IEEE Trans. Electron Devices, Vol. ED-27, No. 6, June, 1980, p. 1045, buffer layer conduction is described.
In a publication by Furutsuka, T., Tsuji, T., and Hasegawa, F., (1978), found in IEEE Trans. MTT-24, p. 512, dipole accumulation layers are discussed.
In a publication by Fukuta, M., Suyama, K., Suzuki, Y., Nakayama, Y., and Ishikawa, H., entitled "Power GaAs MESFET with a High Drain Source Breakdown Voltage," IEEE Trans. Microwave Theory Tech., Vol. MTT-24, pp. 312-317, June, 1976, field distributions were discussed.
In a publication by Ladbrooke, P., and Martin, A. L., entitled "Material and Structure Factors Affecting the Large Signal Operation of GaAs MESFETs," published in the Int. Conf. on Semi-Insulating GaAs, France, 1980, p. 313, traps in the interface region were discussed.
In a publication By Englemann, R. W. H. and Liechti, C. A., (1976), found in IEEE Trans. Electron Devices, p. 1288, differential negative resistance was discussed.
In a publication by Morizane, K., Dosen, M., and Mori, Y., entitled "A Mechanism of Source-Drain Burnout in GaAs MESFETs," (1979), found in the Inst. Phys. Conf. Ser. No. 45, p. 287, thermal burnout in MESFETs was discussed.
Empirically, power delivery at microwave frequencies has been enhanced by reducing parasitics, through recessing the gate to reduce V.sub.K and increase I.sub.f such as discussed by Englemann et al. cited above and by Hasegawa, F., Takayama, Y., Higashisaka, A., Furutsuka, T., and Honjo, K., in a paper entitled "GaAs Power MESFETs with a Simplified Recess Structure," (1978), found in ISSCC Techn. Dig., pp. 118-119.
In a paper by Fukuta, M., et al. published in (1976) in IEEE Trans. Microwave Theory and Tech., MTT-24:312, microwave frequencies were enhanced by inserting N.sup.++ regions at the source and drain contacts to reduce V.sub.K and to modify electric field distributions.
In a publication by Anderson, J. R., Omori, M. and Cooke, F., (1978), IEEE Int. Electron Devices Meeting Dig. Tech. Papers, p. 133, and in a publication by D'Asaro, DiLorenzo, J. V. and Fukuii, H. entitled "Improved GaAs Microwave Field Effect Transistors with Via Connections through the Substrate," (1977), Int. Electron Devices Meeting. Tech. Dig., p. 370, microwave frequencies were enhanced by reducing parasitics through air bridge and via interconnects that reduce source inductance. The above techniques however have had little impact on reducing I.sub.m or increasing V.sub.SD.sup.L.
In a publication by T. Itoh and H. Yanai entitled "Stability of Performance and Interfacial Problems in GaAS MESFET's" in IEEE Trans. on Electron Devices, Vol. ED-27, No. 6, June, 1980, pp. 1037-1044, interface effects in GaAs MESFET's having a 1 .mu.m gate length with and without a buffer layer were investigated.
In U.S. Pat. No. 4,104,672 which issued on Aug. 1, 1978 to DiLorenzo et al., an integrated high power gallium arsenide field effect transistor device is described having a multi-gate structure. FIG. 4 of U.S. Pat. No. 4,104,672 shows gates 30-36.
It is therefore desirable to provide a MESFET having reduced leakage current at pinch-off by depleting the channel substrate interface to provide a MESFET with higher rf power, efficiency, and frequency response and at the same time a lower noise figure.
It is further desirable to provide a MESFET wherein a plurality of openings across the channel permit the gate electrode over the channel to be on the side walls and bottom of the openings which will confine the electrons in the channel region between the openings and provide a depletion layer beneath the channel.
It is further desirable to provide a MESFET of gallium arsenide material with a Schottky barrier gate with indentations in the channel by the gate to permit the gate electrode to wrap around small cross sections of the channel to confine the electrons interior of the channel and to provide a depletion layer beneath the channel.
It is further desirable to provide a MESFET having a plurality of semi-insulating regions which are spaced apart by a predetermined distance in a path across the channel and wherein each region has a predetermined depth at least through the channel and a layer of conductive material deposited thereover along the path to provide a gate electrode wherein the electrons are confined by a depletion layer between the semi-insulating regions and wherein a depletion layer is formed beneath the channel of the MESFET.